Skew compensation and tracking in communications systems

ABSTRACT

Compensation for in-phase (I) and quadrature (Q) timing skew and offset in an optical signal may be achieved based on the correlation between derivatives of I and Q samples in the optical signal. The magnitude of the correlation between derivatives is measured to determine the presence of skew. Correlation between derivatives may be coupled with frequency offset information and/or with trials having additional positive and negative skew to determine presence of skew. Correlations are determined according to pre-defined time periods to provide for continued tracking and compensation for timing skew that may result from, for example, thermal drift.

CROSS REFERENCES

The present application claims the benefit of U.S. Provisional PatentApplication No. 61/435,278, filed on Jan. 22, 2011, entitled “HIGH RATEOPTICAL COMMUNICATION,” which is incorporated herein by reference in itsentirety.

BACKGROUND

The present disclosure relates to systems and methods for compensationof skew between in-phase and quadrature signals in a fiber opticcommunications system.

Fiber optic channels in network communications systems are well knownand are considered effective for data transmission, allowing relativelyhigh bandwidth data communication. Optical fiber used in such channelsis flexible and can be bundled as cables, and is generally considered tobe advantageous for long-distance communications because lightpropagates through the fiber with little attenuation compared toelectrical cables. Typical present day commercial optical fiber systemstransmit data at 10 or 40 Gbit per second. Each fiber can carry multipleindependent channels, each using a different wavelength of light in atechnique known as wavelength-division multiplexing (WDM), therebyenhancing the net data rate of an optical fiber.

As more capacity is continually desired for networks, increased datatransmission rates would be desirable. However, in fiber optic systems,as data rates increase various optical phenomena begin to manifest andact to limit data transmission rates. For example, a propagation skewbetween, for example, in-phase (I) and quadrature (Q) channels in aquadrature phase shift keyed (QPSK) system. Even relatively smallamounts of skew between channels can begin to have a significant impacton system performance at increasing data rates. Such skew can resultfrom, for example, slightly different length communications paths in areceiver, and/or thermal drift in a receiver.

SUMMARY

Methods, systems, and devices are described for compensation of skew inreceived channels of an optical signal. One example of a method of skewcompensation includes receiving a plurality of in-phase and quadraturesamples of a received optical signal, measuring a correlation between aderivative of the in-phase samples and a derivative of the quadraturesamples, and generating a skew correction factor responsive to thecorrelation.

An apparatus for skew compensation may include a skew compensationmodule, a skew correlation module, and a skew update module. The skewcompensation module may receive a plurality of in-phase and quadraturesamples of a received optical signal, receive skew update informationfrom the skew update module, and output skew-compensated in-phase andquadrature samples based on the skew update information. The skewcorrelation module, in various embodiments, receives in-phase andquadrature samples of the received optical signal, and measures acorrelation between a derivative of the in-phase samples and aderivative of the quadrature samples. The correlation between thederivatives provides an indication of skew present in the receivedchannels, and the skew update module may use this information togenerate a skew correction factor responsive to the correlation.

A method of skew compensation may include receiving a plurality ofin-phase and quadrature samples of a received optical signal; measuringa correlation between a derivative of the in-phase samples and aderivative of the quadrature samples; and generating a skew correctionfactor responsive to the correlation.

An apparatus for skew compensation between in-phase and quadraturechannels in an optical signal may include a skew compensation moduleconfigured to receive a plurality of in-phase and quadrature samples ofa received optical signal, receive skew update information, and outputskew-compensated in-phase and quadrature samples based on the skewupdate information; a skew correlation module configured to receivein-phase and quadrature samples of the received optical signal, andmeasure a correlation between a derivative of the in-phase samples and aderivative of the quadrature samples; and a skew update moduleconfigured to receive the measured correlation information and generatea skew correction factor responsive to the correlation.

A system of skew compensation may include means for receiving aplurality of in-phase and quadrature samples of a received opticalsignal, measuring a correlation between a derivative of the in-phasesamples and a derivative of the quadrature samples, and generating askew correction factor responsive to the correlation.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of embodiments ofthe present invention may be realized by reference to the followingdrawings. In the appended figures, similar components or features mayhave the same reference label. Further, various components of the sametype may be distinguished by following the reference label by a dash anda second label that distinguishes among the similar components. If onlythe first reference label is used in the specification, the descriptionis applicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

FIG. 1 is a block diagram of an optical communication system includingcomponents configured according to various embodiments of the invention.

FIG. 2 is a block diagram of an electrical-to-optical unit according tovarious embodiments of the invention.

FIG. 3 is a block diagram of an optical-to-electrical unit according tovarious embodiments of the invention.

FIG. 4 is a block diagram of a demodulator unit according to variousembodiments of the invention.

FIG. 5 is a block diagram of a skew compensation unit according tovarious embodiments of the invention.

FIG. 6 is a block diagram of an exemplary skew compensation unitaccording to an embodiment.

FIG. 7 is a block diagram of an exemplary skew compensation unitaccording to an embodiment.

FIG. 8 is a block diagram of an exemplary skew compensation unitaccording to an embodiment.

FIG. 9 is a block diagram of an exemplary skew compensation unitaccording to an embodiment.

FIG. 10 is a flow chart of a method for skew compensation according tovarious embodiments of the invention.

FIG. 11 is a flow chart of another method for skew compensationaccording to various embodiments of the invention.

DETAILED DESCRIPTION

This description provides examples, and is not intended to limit thescope, applicability or configuration of embodiments of the invention.Rather, the ensuing description will provide those skilled in the artwith an enabling description for implementing embodiments of theinvention. Various changes may be made in the function and arrangementof elements.

Thus, various embodiments may omit, substitute, or add variousprocedures or components as appropriate. For instance, it should beappreciated that the methods may be performed in an order different thanthat described, and that various steps may be added, omitted orcombined. Also, aspects and elements described with respect to certainembodiments may be combined in various other embodiments. It should alsobe appreciated that the following systems, methods, devices, andsoftware may individually or collectively be components of a largersystem, wherein other procedures may take precedence over or otherwisemodify their application.

Systems, devices, methods, and software are described for an opticalcommunication system that utilizes fiber optic optical cables as a datatransmission medium. An example of an optical data transport system 100is illustrated in FIG. 1. In this embodiment, the optical data transportsystem 100 includes a data source that provides data to a digital codingand modulation unit 105. The data source may include any of a number ofcommon data sources, such as a user telecommunications device, a cableoperator head-end unit, a telecommunications provider central office, acomputer server, or a network attached storage system, to name but a fewexamples. In many embodiments, the data source generates significantquantities of data to be transported across the optical data transportsystem 100. The digital coding and modulation unit 105 receives thisdata, and performs framing, forward error correction coding, andmodulation functions on the data. The electrical-to-optical (E-O) unit110 transforms the data into optical signals, and transmits opticalsignals containing the data via a fiber connection 115. The fiberconnection 115 may include well known components of such connections,including a fiber optic cable. An optical-to-electrical (O-E) unit 120receives the optical signal from the fiber connection 115, andtransforms the data into the electrical domain. The digital demodulationand decoding unit 125 receives the digitized version of the opticalsignal and performs demodulation, forward error correction decoding, andde-framing functions on the data from the optical signal. The digitaldemodulation and decoding unit 125 may then output the data (e.g., to auser telecommunications device, a cable operator head-end unit, atelecommunications provider central office, a computer server, or anetwork attached storage system).

FIG. 2 illustrates a digital coding and modulation unit 105-a. In theillustrated embodiment, the digital coding and modulation unit 105-aincludes a data transport layer framer module 205, an FEC coder module210, a modulator module 215, and a transmitter fiber interface module220. The data transport layer framer module 205 may place the datareceived from the data source into packet frames for transmission. Thepacket frames may conform to one of many common protocols for packetframes used in optical communications systems which commonly include aheader and a payload, and possibly a trailer, such as a CRC. As is wellunderstood, the header may be interleaved with the payload duringtransmission, depending upon the particular protocol being used foroptical transmission. The FEC coder module 210 calculates and addsforward error correction (FEC) information to the frames of datareceived from the data transport layer framer module 205. The particulartype of FEC information of various embodiments generally includessystematically generated redundant error-correcting code (ECC) data thatis transmitted along with the frames. There may be differential encodingused in some embodiments. The modulator module 215 modulates the framesand FEC information, forwarding the data to a transmitter fiberinterface module 220. The transmitter fiber interface module 220 mayforward the modulated data to the E-O module where it may be transmittedin the optical domain via a dual-polarity (dual-pole) quadraturephase-shift-keying (QPSK) modulation, resulting in four parallel opticalstreams. Other modulation schemes may be used in other examples, aswell.

As illustrated in FIG. 3, a digital demodulation and decoding unit 125-amay include a number of modules, as well. In this embodiment the digitaldemodulation and decoding unit 125-a includes a receiver fiber interfacemodule 305, a demodulator module 310, an FEC decoder module 315, and adata transport layer de-framer module 320. The receiver fiber interfacemodule 305 is the interface from the O-E unit 120. The receiver fiberinterface module 305 provides electrical signals to a demodulator module310. Various embodiments of the demodulator module 310 will be discussedin further detail below. The information from the demodulator module 310is provided to the FEC decoder module 315 which decodes and may correcttransmission errors identified from error-correcting code. The FECdecoder module 315 provides decoded data to the data transport layerde-framer module 320, which de-frames the data from the signal accordingto the particular protocol used in the optical transmission, andprovides output data. The data output may be, for example, a user or anyreceiving system.

These components may, individually or collectively, be implemented withone or more Application Specific Integrated Circuits (ASICs) adapted toperform some or all of the applicable functions in hardware.Alternatively, the functions may be performed by one or more otherprocessing units (or cores), on one or more integrated circuits. Inother embodiments, other types of integrated circuits may be used (e.g.,Structured/Platform ASICs, Field Programmable Gate Arrays (FPGAs) andother Semi-Custom ICs), which may be programmed in any manner known inthe art. The functions of each unit may also be implemented, in whole orin part, with instructions embodied in a memory, formatted to beexecuted by one or more general or application-specific processors.

Referring now to FIG. 4, a demodulator unit 400 is described. This maybe the demodulator unit 310 of FIG. 3. In this example, two polarizationcomponents are received, one horizontal component (H) and one verticalcomponent (V). Each of the H and V components includes both an in-phase(I) component and a quadrature (Q) component. For reference, the twocomponents in the horizontal polarization are referred to as HI(horizontal in-phase component) and HQ (horizontal quadraturecomponent). Similarly, the two components in the vertical polarizationare referred to as VI (vertical in-phase component) and VQ (verticalquadrature component). The demodulator unit 400 processes the digitizedsamples of the I and Q components of the two polarization components torecover the transmitted data. At the input, the demodulator unit 400accepts the four parallel streams carrying HI, HQ, VI and VQ samples. Inone embodiment, each stream contains multiple samples per clock. At itsoutput the demodulator may provide demodulated hard-decision data(although in other examples, soft-decision data may be provided) to theFEC decoder module (e.g., FEC decoder module 315 of FIG. 3). Thedemodulator unit 400 may identify the beginning of an FEC frame.Additionally, in some embodiments the demodulator unit 400 receivesfeedback signals from the FEC decoder module 315 regarding theconvergence status for error correction.

In some embodiments, the demodulator unit 400 is implemented as anapplication specific integrated circuit (ASIC) that includes a number offunctional modules. In such embodiments, the demodulator unit 400 mayhave a control and monitor interface bus 405 connected to a hostprocessor 410 allowing for configuration of demodulator parameters(filter coefficients, loop gains, etc.) and extraction of demodulatorstatus. With continuing reference to FIG. 4, several of the sub-moduleswithin the demodulator unit 400 of various embodiments are described. Inthis embodiment, a quadrature error filter (QEF) module 415 provides acollection of data formatting, error detection and correction functions.In one embodiment, input data samples are expected to be inbinary-offset/offset-binary format and are converted to a two'scomplement (2C) format for processing within a digital signal processor.The incoming HI, HQ, VI and VQ streams, in some embodiments, also can beindependently swapped and inverted if needed, allowing for any designissues that might translate into an accidental inversion or IQ swap.Each data stream of these various embodiments may be processed to removepolarization skew (between H and V poles) as well as I-Q skew within apole. The QEF module 415 may provide for detection and removal of fourtypes of quadrature signal errors: I/Q Skew, DC bias, I/Q amplitudeimbalance, and I/Q phase imbalance. All four error detectors may beindependently enabled or disabled, in some embodiments, via theprocessor interface, and the detected error values are output as statusvalues via this same interface. The QEF module 415 may also output again control signal that may be used by other components of the system.

The QEF module 415 is connected to a frequency offset removal module420. The frequency offset removal module 420 in one example performs afrequency rotation on the data samples coming out of the QEF module 415.The amount of frequency rotation is controlled by a frequency errorinput that is sourced by a carrier frequency acquisition and tracking(CFAT) module 425. Such frequency offset removal function may removeresidual frequency left from the local oscillator (LO) laser tuning inthe optical domain. A bulk dispersion compensation module 430 removesbulk chromatic dispersion from the horizontal and vertical polarizationchannels. The compensation may be applied via a filter in the frequencydomain. The amount of correction may be controlled by the chromaticdispersion filter inputs that are derived outside of the demodulatormodule 400 and provided via the host processor 410 and control andmonitor interface bus 405, in this embodiment.

A matched filter decimator (MFD) module 435 may implement aninterpolation function that provides decimation on samples taken attwo+ε times the symbol rate. In one embodiment, each of the four datastreams has an independent bank of FIR filters with selectedcoefficients. The incoming data is processed through the filter banks toproduce two samples per symbol out for each data stream. Data samplesare gathered and assembled into blocks of fixed numbers of samples perstream per clock by a sample block assembler. The assembly function maybe identical for the I and Q streams in each polarization, so oneassembly block may service two streams. A PMD/PDL compensation module440 may utilize adaptive equalization to compensate forcross-polarization interference, IQ channel interference, adjacentsymbol interference introduced by PMD and PDL in optical channel andother residual impairments, such as residual chromatic as mentionedabove. On one embodiment, an adaptive equalizer takes in data at one ortwo samples/symbol from the MFD and processes the data through a bank ofFIR filters with adaptive filter tap coefficients.

In some embodiments, a symbol timing acquisition and tracking (STAT)module 445 may estimate symbol timing using an early/late symbol radiusmatching scheme and PI controller, and generate an error signal tocorrect symbol timing. This STAT module 445, in an embodiment, also hasa symbol timing lock detection mechanism that outputs a symbol lockindicator. In various embodiments, there are two sets of gains for thePI controller (wide band for acquisition and narrow band for tracking).When not in timing lock, the wideband gains may be used, otherwise, thenarrowband gains may be used. The STAT module 445 may perform symboltiming acquisition and tracking of a portion of the optical signal afterthe PMD/PDL compensation module 440 compensates for interference causedby PMD and PDL and before carrier phase recovery on the portion of theoptical signal.

The CFAT module 425 may be responsible for acquiring, as well astracking, carrier frequency. Carrier frequency acquisition is achievedusing one of a number of techniques, such as through fast Fouriertransform (FFT) with appropriate averaging and peak frequency componentdetection. The CFAT module 425 may provide a frequency error input tothe frequency offset removal module 420. The CFAT module 425, in someembodiments, also provides local oscillator (LO) frequency offset outputthat may be used with data from the FEC frame synchronization andinterface module 460. A carrier phase recovery (CPR) module 450 may usea feed-forward algorithm with a block phase estimator and a phaserotation function to remove residual frequency and phase errors. The CPRmodule 450 may operate on the on-time data samples produced by the PMDcompensation module. A differential decoder module 455 may beresponsible, in various embodiments, for accepting symbol streams fromthe CPR module 450 (e.g., at 1 sample per symbol). The differentialdecoder module 455 may be configured to differentially decode the signaland provide the decoded output (e.g., a hard-decision output datastream) to the FEC frame synchronization and interface module 460. TheFEC frame synchronization and interface module 460 processes data toachieve frame synchronization, and may include three functional blocksfor data alignment, frame sync detection, and clock transfer. The FECframe synchronization and interface module 460 may be configured toskew, swap, and rotate received channels with respect to each other.

With reference now to FIG. 5, a block diagram of a skew compensationsystem 500 according to various embodiments is described. In thisexemplary diagram, an input is received at skew compensation module 505.The input may include two or more input channels, such as an in-phaseand a quadrature channel of an QPSK input. In one embodiment, the inputreceives digitized samples of in-phase and quadrature channels for bothhorizontal and vertical polarity received optical signals, therebyresulting in four separate received channels. Throughout thisdescription, reference is generally made to in-phase (I) and quadrature(Q) channels, and it is to be understood that multiple sets of such Iand Q channels may be received. Furthermore, the concepts describedherein are applicable to other types of input channels in which areceiver, or transmitter, may impart some amount of timing skew intosuch channels. Accordingly, while many of the above examples aredirected to optical systems, it is to be understood that skewcompensation concepts as described herein are applicable to anyquadrature demodulator including demodulators used in both optical andnon-optical applications. Skew compensation systems such as in FIG. 5may be included in a receiver prior to demodulation of a receivedsignal, or may be incorporated as part of the demodulator in such asystem. In some embodiments, the components of FIG. 5 are included inthe quadrature error filter module 415 of FIG. 4.

With continuing reference to FIG. 5, the skew compensation module 505receives the input and may delay the output of one or more of the inputchannels relative to other channels before providing the channels to DC,phase, and amplitude compensation module 510. The DC, phase, andamplitude compensation module 510 analyzes the incoming signal channelsand adjusts any DC offset, phase imbalance, and amplitude imbalance inthe received input channels, so the channels substantially match eachother and have values that correspond to the design of the remainingportions of the demodulator. The DC, phase, and amplitude compensationmodule 510 outputs the skew compensated and DC, phase, and amplitudecompensated channels to the next component of the demodulator, such asthe frequency offset removal module 420 of FIG. 4. A filter module 515is connected to the output of the DC, phase, and amplitude compensationmodule 510 and receives samples of the output channels. The filtermodule 515 may be a simple digital filter. The filter module 515 of someembodiments provides an output that corresponds to the first derivativeof the I and Q samples. The current disclosure recognizes that in thepresence of a time skew and a frequency offset there is a non-zerocorrelation between first derivatives of I and Q samples. A correlationmodule 520 is connected to the output of filter module 515, anddetermines a correlation between I and Q samples, the presence ofcorrelation indicating that timing between channels may be skewed. Insome embodiments, the correlation module 520 determines correlationbetween derivatives according to a relation between the correlation,frequency offset and skew according to the formula:C=k*F*S*P, where:

k is an implementation-based constant

C is the average correlation of I and Q derivatives;

F is the frequency offset (normalized to sample rate);

S is I to Q time skew (normalized to sample rate); and

P is input signal power.

In some embodiments, two samples per symbol are averaged together over apredetermined time period to determine an average correlation. In oneembodiment, the system operates at 20 Gsps, and skew is tracked to 0.5%of a symbol period (or 1% of sample period), k is determined to be 4,and it is known that input frequency error will on average be no smallerthat 0.5% of the sample rate or 1% of the symbol rate, resulting in amean correlation of 4*(0.005*2)*0.01=4e−4. If a worst-case of 7 dB SNRis assumed, an expected worst-case variance on this measurement would be0.5. Accordingly, if it is desired that the variance of the estimate be25% of actual (i.e. 1e−4) then average (1/1e−4)^2=1e8, or about 5milli-seconds (at 20 Gsps). In other embodiments, the correlation module515 may determine the product of the first derivative of the I and Qsamples and output a correlation information based on the magnitude ofthe product, which may be averaged over a predetermined time period.

The correlation module 520 provides the correlation information to askew update module 525, which provides a skew update to the skewcompensation module 505. In some embodiments, the correlation module 520includes a low pass filter that outputs a filtered version of theaverage correlation that is used as an input to update filter taps ofone or more finite impulse response (FIR) filters of the skewcompensation module 505. In other embodiments, the correlation module520 includes controller, such as a PI controller, that receives theaverage correlation and outputs filter tap updates for one or morefinite FIR filters of the skew compensation module 505.

In other embodiments, a small amount of positive or negative skew isadded to I and Q samples, and correlation between first derivatives ofsamples of the samples are evaluated to determine if the positive ornegative skew decreased correlation. One exemplary embodiment of such asystem is illustrated in the block diagram of FIG. 6. The skewcompensation system 600 in FIG. 6 includes a DC offset compensationmodule 605 that receives I and Q input channels. A DC compensatedversion of the I and Q channels are provided to skew compensation module610. The skew compensation module 610 adjusts the I and/or Q channels tocompensate for any detected skew, and provides the compensated output tophase and amplitude compensation module 615. The phase and amplitudecompensation module 615 compensates for phase and amplitude imbalance,and provides the compensated I and Q channels to the next component ofthe demodulator. In this embodiment, a correlation module 620 isconnected to each of the I and Q outputs and determines a correlationbetween derivatives of samples of the I and Q channels. The correlationmodule 620 of some embodiments determines a magnitude of the product ofthe first derivatives of the I and Q samples and passes the resultsthrough an infinite impulse response (IIR) low-pass filter. Thecorrelation module 620 provides correlation information to skew updatemodule 625.

As noted above, the system 600 of FIG. 6 also adds some amount ofpositive or negative skew to I and Q samples. Negative skew module 630adds a known amount of negative skew to the I and Q channels. In oneembodiment, the negative skew module skews samples of the I and Qchannels by 0.5 percent of the sample period, although it will beunderstood that other amounts of skew may be implemented. A phase andamplitude compensation module 635 compensates for phase and amplitudeoffset, and provides the compensated and skewed I and Q channels to acorrelation module 640. The correlation module 640 determines acorrelation between derivatives of the negative skewed samples of the Iand Q channels. The correlation module 640, similarly to correlationmodule 620, may determine a magnitude of the product of the firstderivatives of the I and Q samples and pass the results through aninfinite impulse response (IIR) low-pass filter. The correlation module640 provides negative skew correlation information to skew update module625.

Positive skew module 645 of FIG. 6 adds a known amount of positive skewto the I and Q channels. In one embodiment, the positive skew moduleskews samples of the I and Q channels by 0.5 percent of the sampleperiod, although it will be understood that other amounts of skew may beimplemented. Phase and amplitude compensation module 650 compensates forphase and amplitude offset, and provides the compensated and skewed Iand Q channels to a correlation module 655. The correlation module 655determines a correlation between derivatives of the positive skewedsamples of the I and Q channels. The correlation module 655, similarlyto correlation modules 620 and 640, may determine a magnitude of theproduct of the first derivatives of the I and Q samples and pass theresults through an IIR low-pass filter. The correlation module 655provides positive skew correlation information to skew update module625.

Skew update module 625 of FIG. 6 receives correlation information fromeach of the three correlation modules 620, 640, and 655, and determineswhich of the three outputs has the lowest magnitude value. The signalwith the lowest magnitude value indicates the lowest correlation betweenthe I and Q channels, and also the lowest amount of skew between thechannels. If either the positively skewed channel or negatively skewedsamples have correlation values that are lower than the samples that donot have additional skew, the skew update module 625 provides an updateto the skew compensation module 610 to adjust skew in the I and Qchannels. In one embodiment, the skew update module 625 is a statemachine that periodically resets the IIR filters of the correlationmodules 620, 640, 655, waits a predetermined amount of time, comparesthe three filtered correlation values, and updates the skew compensationmodule 610 based on the received correlation values.

FIG. 7 is a block diagram of another embodiment in which a small amountof positive or negative skew is added to I and Q samples, andcorrelation between first derivatives of samples of the samples areevaluated to determine if the positive or negative skew decreasedcorrelation. The skew compensation system 700 in FIG. 7 receives I and Qchannels directly at skew compensation module 705. The skew compensationmodule 705 adjusts the I and/or Q channels to compensate for anydetected skew, and provides the compensated output to a DC, phase andamplitude compensation module 710. The DC, phase and amplitudecompensation module 710 compensates for DC offset and phase andamplitude imbalance, and provides the compensated I and Q channels tothe next component of the demodulator. In this embodiment, a correlationmodule 715 is connected to each of the I and Q outputs of the skewcompensation module 705 and determines a correlation between derivativesof samples of the I and Q channels. The correlation module 735 of someembodiments determines a magnitude of the product of the firstderivatives of the I and Q samples and passes the results through an IIRlow-pass filter. The correlation module 715 provides correlationinformation to skew update module 720.

As noted above, the system 700 of FIG. 7 also adds some amount ofpositive or negative skew to I and Q samples. Negative skew module 725adds a known amount of negative skew to the I and Q channels, such as0.5 percent of the sample period, and provides the skewed I and Qchannels to a correlation module 730. The correlation module 730determines a correlation between derivatives of the negative skewedsamples of the I and Q channels. The correlation module 730, similarlyto correlation module 715, may determine a magnitude of the product ofthe first derivatives of the I and Q samples and pass the resultsthrough an IIR low-pass filter. The correlation module 730 providesnegative skew correlation information to skew update module 720.Similarly, positive skew module 735 adds a known amount of positive skewto the I and Q channels, such as 0.5 percent of the sample period, andprovides the skewed I and Q channels to a correlation module 740. Thecorrelation module 740 determines a correlation between derivatives ofthe positive skewed samples of the I and Q channels and, similarly tocorrelation modules 715 and 730, may determine a magnitude of theproduct of the first derivatives of the I and Q samples and pass theresults through an IIR low-pass filter. The correlation module 740provides positive skew correlation information to skew update module720.

Skew update module 720 of FIG. 7 receives correlation information fromeach of the three correlation modules 715, 730, and 740, and determineswhich of the three outputs has the lowest magnitude value. The signalwith the lowest magnitude value indicates the lowest correlation betweenthe I and Q channels, and also the lowest amount of skew between thechannels. If either the positively skewed channel or negatively skewedsamples have correlation values that are lower than the samples that donot have additional skew, the skew update module 720 provides an updateto the skew compensation module 705 to adjust skew in the I and Qchannels. In one embodiment, the skew update module 720 is a statemachine that periodically resets the IIR filters of the correlationmodules 715, 730, and 740, waits a predetermined amount of time,compares the three filtered correlation values, and updates the skewcompensation module 705 based on the received correlation values.

In still other embodiments, the system may have information related tofrequency offset from a baseband frequency for the I and Q channels.This frequency offset information may be used in a manner similarly asdescribed above, to determine a skew update. A block diagram of skewcompensation system 800 that uses frequency offset information isillustrated in FIG. 8. In the system 800 of FIG. 8, I and Q channels arereceived at a bulk skew compensation module 805. The bulk skewcompensation module 805 compensates for skew that is more than half of asample period, and is configured during an initial calibration of thesystem 800. A DC, phase and amplitude compensation module 810compensates for DC offset, and phase and amplitude imbalance in thereceived I and Q channels. A fine skew compensation module 815 receivesthe DC, phase and amplitude compensated I and Q channels, andcompensates for detected skew that was not removed at the bulk skewcompensation module 805. The fine skew compensation module 815 outputsthe skew compensated and DC, phase, and amplitude compensated I and Qchannels to the next component of the demodulator, such as the frequencyoffset removal module 420 of FIG. 4.

A correlation module 820 of FIG. 8 is connected to the output of the DC,phase, and amplitude compensation module 810 and receives samples of theoutput channels and also received frequency offset information, such asfrom another component of the demodulator. The correlation module 820determines a correlation between I and Q samples, the presence ofcorrelation indicating that timing between channels may be skewed. Insome embodiments, the correlation module 820 determines correlationbetween derivatives of samples of the I and Q channels as a product ofthe frequency offset, and derivatives of the I and Q samples. Thecorrelation module 820 provides the correlation information to skewupdate module 825, which provides a skew update to the fine skewcompensation module 815. In some embodiments, the skew update moduleincludes an IIR low pass filter that outputs a filtered version of theaverage correlation that is used as an input to update filter taps ofone or more finite impulse response (FIR) filters of the fine skewcompensation module 815.

With reference now to FIG. 9, block diagram of another skew compensationsystem 900 that uses frequency offset information is illustrated. In thesystem 900, I and Q channels are received at a skew compensation module905. The skew compensation module 905 compensates for detected skew inthe I and Q channels, and provides compensated signals to DC, phase andamplitude compensation module 910. DC, phase and amplitude compensationmodule 910 compensates for DC offset, and phase and amplitude imbalancein the received I and Q channels and outputs the skew compensated andDC, phase, and amplitude compensated I and Q channels to the nextcomponent of the demodulator, such as the frequency offset removalmodule 420 of FIG. 4. A correlation module 915 is connected to theoutput of the DC, phase, and amplitude compensation module 910 andreceives samples of the output channels and also received frequencyoffset information, such as from another component of the demodulator.In some embodiments, the frequency offset information includes apositive or negative sign of the frequency error. The correlation module915 determines a correlation between I and Q samples, the presence ofcorrelation indicating that timing between channels may be skewed. Insome embodiments, the correlation module 915 determines correlationbetween derivatives of samples of the I and Q channels as a product ofthe sign of the frequency offset, and derivatives of the I and Qsamples. The correlation module 915 provides the correlation informationto skew update module 920, which provides a skew update to the skewcompensation module 905. In some embodiments, the skew update module 920includes a PI controller that outputs an update signal based on theaverage of the received correlation information, the output used toupdate filter taps of one or more finite impulse response (FIR) filtersof the skew compensation module 905.

With reference now to FIG. 10, a flow chart diagram 1000 of theoperational steps for compensating skew of various embodiments isdescribed. Initially, at block 1005, a plurality of in-phase andquadrature samples of an optical signal are received. At block 1010 acorrelation between derivatives of the in-phase and quadrature samplesis measured. A skew correction factor is generated, at block 1015,responsive to the correlation. The skew correction factor may be used tocompensate for skew that is detected based on the measured correlation.

FIG. 11 includes a flow chart diagram 1100 of the operations steps foroperational steps for compensating skew according to other embodiments.Initially, at block 1105, a plurality of in-phase and quadrature samplesof an optical signal are received. At block 1110 a correlation betweenmultiple derivatives of the in-phase and quadrature samples is measured.A skew correction factor is generated, at block 1115, responsive to thecorrelations. The skew correction factor is used to compensate inreceived channels, according to block 1120. At block 1125, DC offset,phase and amplitude imbalances are compensated. Finally, at block 1130,skew compensated, and DC, phase, and amplitude compensated signals areoutput. The compensated signal may be output to the next component ofthe demodulator for the optical signal, such as a frequency offsetremoval module 420 of FIG. 4.

As will be readily understood, the components and modules described withreference to various embodiments above may, individually orcollectively, be implemented with one or more Application SpecificIntegrated Circuits (ASICs) adapted to perform some or all of theapplicable functions in hardware. Alternatively, the functions may beperformed by one or more other processing units (or cores), on one ormore integrated circuits. In other embodiments, other types ofintegrated circuits may be used (e.g., Structured/Platform ASICs, FieldProgrammable Gate Arrays (FPGAs) and other Semi-Custom ICs), which maybe programmed in any manner known in the art. The functions of each unitmay also be implemented, in whole or in part, with instructions embodiedin a memory, formatted to be executed by one or more general orapplication-specific processors.

It should be noted that the methods, systems and devices discussed aboveare intended merely to be examples. It must be stressed that variousembodiments may omit, substitute, or add various procedures orcomponents as appropriate. For instance, it should be appreciated that,in alternative embodiments, the methods may be performed in an orderdifferent from that described, and that various steps may be added,omitted or combined. Also, features described with respect to certainembodiments may be combined in various other embodiments. Differentaspects and elements of the embodiments may be combined in a similarmanner. Also, it should be emphasized that technology evolves and, thus,many of the elements are exemplary in nature and should not beinterpreted to limit the scope of embodiments of the invention.

Specific details are given in the description to provide a thoroughunderstanding of the embodiments. However, it will be understood by oneof ordinary skill in the art that the embodiments may be practicedwithout these specific details. For example, well-known circuits,processes, algorithms, structures, and techniques have been shownwithout unnecessary detail in order to avoid obscuring the embodiments.

Also, it is noted that the embodiments may be described as a processwhich is depicted as a flow diagram or block diagram. Although each maydescribe the operations as a sequential process, many of the operationscan be performed in parallel or concurrently. In addition, the order ofthe operations may be rearranged. A process may have additional stepsnot included in the figure.

Furthermore, embodiments may be implemented by hardware, software,firmware, middleware, microcode, hardware description languages, or anycombination thereof. When implemented in software, firmware, middlewareor microcode, the program code or code segments to perform the necessarytasks may be stored in a computer-readable medium such as a storagemedium. Processors may perform the necessary tasks.

Having described several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of theinvention. For example, the above elements may merely be a component ofa larger system, wherein other rules may take precedence over orotherwise modify the application of the invention. Also, a number ofsteps may be undertaken before, during, or after the above elements areconsidered. Accordingly, the above description should not be taken aslimiting the scope of the invention.

What is claimed is:
 1. A method of skew compensation, the methodcomprising: receiving a plurality of in-phase and quadrature samples ofa received optical signal; measuring a correlation between a derivativeof the in-phase samples and a derivative of the quadrature samples,wherein measuring the correlation comprises: receiving frequency offsetinformation corresponding to a difference between the frequency of thereceived samples and a baseband modulation frequency; and multiplyingthe frequency offset information by the derivative of the in-phasesamples and the derivative of the quadrature samples; and generating askew correction factor responsive to the correlation.
 2. The method ofclaim 1, wherein the measuring comprises measuring a pluralitycorrelations between one or more derivatives of the in-phase samples andone or more derivatives of the quadrature samples; and the generatingcomprises calculating the skew correction factor based on the pluralityof measured correlations.
 3. The method of claim 2, wherein calculatinga skew correction factor comprises calculating an average of theplurality of measured correlations, and calculating the skew correctionfactor based on a prior skew correction factor and the calculatedaverage.
 4. The method of claim 1, further comprising: time-shifting anin-phase sample and a corresponding quadrature sample by a predeterminedamount; measuring a time-shifted correlation between a derivative of thetime-shifted samples; and comparing the correlation and time-shiftedcorrelation; and wherein generating a skew correction factor comprisesgenerating a skew correction factor responsive to the comparedcorrelations.
 5. The method of claim 1, wherein the frequency offsetinformation comprises a magnitude of the frequency offset.
 6. The methodof claim 1, wherein the frequency offset information comprises a sign ofthe frequency offset.
 7. An apparatus for skew compensation betweenin-phase and quadrature channels in an optical signal, comprising: askew compensation module configured to receive a plurality of in-phaseand quadrature samples of a received optical signal, receive skew updateinformation, and output skew-compensated in-phase and quadrature samplesbased on the skew update information; a skew correlation moduleconfigured to receive in-phase and quadrature samples of the receivedoptical signal, and measure a correlation between a derivative of thein-phase samples and a derivative of the quadrature samples; and a skewupdate module configured to receive the measured correlation informationand generate a skew correction factor responsive to the correlation,wherein the skew correlation module is further configured to receivefrequency offset information corresponding to a difference between thefrequency of the received samples and a baseband modulation frequencyand measure correlation between a derivative of the in-phase samples anda derivative of the quadrature samples based on a product of thefrequency offset information and the derivatives of the in-phase andquadrature samples.
 8. The apparatus of claim 7, wherein the skew updatemodule is configured to (i) receive a plurality of measured correlationsbetween a derivative of the in-phase samples and a derivative of thequadrature samples, and (ii) calculate a skew correction factor based onthe plurality of measured correlations.
 9. The apparatus of claim 8,wherein the skew update module is configured to calculate an average ofthe plurality of measured correlations, and calculate the skewcorrection factor based on a prior skew correction factor and thecalculated average.
 10. The apparatus of claim 7, wherein the skewcorrelation module comprises: a sampling module configured to providein-phase and quadrature trial samples that include a predeterminedamount of positive or negative skew; and a measurement module configuredto measure (i) a first correlation between a derivative of the in-phasesamples and a derivative of the quadrature samples, and (ii) a secondcorrelation between a derivative of the in-phase and quadrature trialsamples.
 11. The apparatus of claim 10, wherein the skew update moduleis configured to receive the first and second correlations and generatethe skew correction factor based on the magnitude of the first andsecond correlations.
 12. The apparatus of claim 10, wherein the samplingmodule is configured to provide a first set of in-phase and quadraturetrial samples that include a predetermined amount of positive skew, andto provide a second set of in-phase and quadrature trial samples thatinclude a predetermined amount of negative skew; and wherein themeasurement module configured to measure (i) a first correlation betweena derivative of the in-phase samples and a derivative of the quadraturesamples, (ii) a second correlation between a derivative of the first setof trial samples, and (iii) a third correlation between a derivative ofthe second set of trial samples.
 13. The apparatus of claim 12, whereinthe skew update module is configured to receive the first, second, andthird correlations and generate the skew correction factor based on themagnitude of the first, second, and third correlations.
 14. Theapparatus of claim 13, wherein the skew correction factor is based onthe minimum of the first, second, and third correlations.
 15. Theapparatus of claim 7, wherein the frequency offset information comprisesa magnitude of the frequency offset.
 16. The apparatus of claim 7,wherein the frequency offset information comprises a sign of thefrequency offset.
 17. A system of skew compensation, the systemcomprising: means for receiving a plurality of in-phase and quadraturesamples of a received optical signal; means for measuring a correlationbetween a derivative of the in-phase samples and a derivative of thequadrature samples; and means for generating a skew correction factorresponsive to the correlation, wherein the means for measuring acorrelation comprises: means for receiving frequency offset informationcorresponding to a difference between the frequency of the receivedsamples and a baseband modulation frequency; and means for multiplyingthe frequency offset information by the derivative of the in-phasesamples and the derivative of the quadrature samples.
 18. The system ofclaim 17, wherein the means for measuring comprises means for measuringa plurality correlations between one or more derivatives of the in-phasesamples and one or more derivatives of the quadrature samples; and themeans for generating comprises means for calculating the skew correctionfactor based on the plurality of measured correlations.
 19. The systemof claim 18, wherein the means for calculating a skew correction factoris configured to calculate an average of the plurality of measuredcorrelations, and calculate the skew correction factor based on a priorskew correction factor and the calculated average.
 20. The system ofclaim 17, further comprising: means for time-shifting an in-phase sampleand a corresponding quadrature sample by a predetermined amount; meansfor measuring a time-shifted correlation between a derivative of thetime-shifted samples; and means for comparing the correlation andtime-shifted correlation; and wherein the means for generating a skewcorrection factor is configured to generate a skew correction factorresponsive to the compared correlations.
 21. The system of claim 17,wherein the frequency offset information comprises a magnitude of thefrequency offset.
 22. The system of claim 17, wherein the frequencyoffset information comprises a sign of the frequency offset.